It is known that repair shops conventionally use machines for performing repairs and maintenance of vehicle wheels, such as for example tire changing machines, which are designed to fit and remove the tire on and from the rim of the wheel.
These repair shop machines are commonly provided with fixtures for fastening the rim of the wheel being worked, which are turned about a working axis by way of conventional motor means; in practice, the placement of the rim coaxially to said work axis allows to turn the wheel about its own axis.
One particular type of these fixtures is constituted by a self-centering spindle, which has a substantially cylindrical frame and can be associated with the motor means of the machine coaxially to the work axis.
Said frame supports four movable arms, which are arranged symmetrically around the work axis and are mutually offset by 90° of rotation about said axis, each arm supporting in turn a claw for gripping the rim of the wheel.
In particular, the arms are associated with the frame so that they can rotate about respective rotation axes, which are mutually coplanar and perpendicular to the work axis, so as to move the claws away from or toward said axis, respectively in order to push them against the internal cylindrical portion of the rim and allow to grip it, and to separate them from said portion of the rim and release it.
Said known spindles are susceptible of further improvements aimed in particular at increasing their operating efficiency.
The four claws supported on the corresponding arms in fact sometimes do not ensure perfect stability of the rim on the spindle; the wheel can therefore be subjected to undesirable relative movements with respect to the fixture for fastening to the machine.
In this regard, it is noted that even slight relative movements can compromise the correct placement of the wheel with respect to the work axis, with the risk of compromising the correct execution of work.
Besides, it is not possible to exclude the possibility of unexpected separations of the rim from the spindle, which can jeopardize the safety of the personnel assigned to work that is present in the vicinity of the machine.
Moreover, it is noted that if the work axis is substantially horizontal, the arms that support the wheel work substantially in a cantilevered fashion with respect to the frame with which they are associated and can be subjected to even substantial flexural loads, especially if they are used to support very heavy and bulky wheels, such as the wheels of trucks, tractors or the like.